The stylus and tablet are well known devices by means of which a user can interact with data displayed on a display screen. Several stylus and tablet combinations are described, for example, in a textbook `Principles of Interactive Computer Graphics` by Newman and Sproull, 2nd Edition, Section 11-3, `Tablets`. A stylus and tablet may be used in any of several ways to input data to a graphics display system to which it is attached. However, whether the stylus is being used to point to locations on the tablet, for example, to select items from a `menu` of items overlaying the tablet surface, or to digitize by tracing or drawing, curves or diagrams for display on the screen, or to control the position of a graphics cursor on the screen, the information supplied to the system is always the same, and is in the form of signals representing the coordinate values of the position of the stylus tip with respect to the tablet surface.
A stylus typically incorporates a pressure-sensitive switch which closes when the user pushes the stylus against the tablet, or alternatively a button on the side of the stylus which can be operated at any time by the user, either of which are used to indicate to the application program being run on the graphics system that the stylus is at a position of interest.
The sensing mechanism for generating the stylus position signals varies from tablet to tablet, as will be apparent by reference to the aforementioned text book, but is most commonly electrical in nature. Upon receipt of the position signals identified by the user as of interest, the graphics system usually responds by displaying a cursor on the screen at a position corresponding to that of the stylus on the tablet, thus providing visual feedback to the user. In practice, the sensing mechanism for generating the coordinate signals is capable of detecting the presence of a stylus while it is still some distance above the tablet surface beyond which the tablet is incapable of reliably measuring the X and Y coordinates of the tip. However, once inside this notional boundary, the position signals derived by the tablet and stylus sensing mechanism have risen above noise level thresholds and can be reliably used by the system. The position of the boundary depends upon the sensitivity of the sensing mechanism and the signal level thresholds set by the logic sensing circuits within the graphics system itself, but typically lies between 5 mm and 10 mm above the tablet surface. Since a stylus tip inside the boundary is detectable by the system and its position over the surface of the tablet can be tracked, whereas a stylus outside the boundary is undetectable by the system, the boundary is referred to as the `in-presence` boundary of the tablet.
In some applications and workstation configurations, it is desirable to give the user an indication of whether the stylus is `in-presence` within the `in-presence` boundary or `out of presence` beyond the `in-presence` boundary and thus whether or not the workstation is tracking the stylus tip. Digitizing applications often make use of the `in-presence` or out-of-presence condition to distinguish between `draw` and `move` instructions and, as the user may be making as many as twenty or thirty pointings per minute, it is highly desirable to give an indication of which operation the workstation is performing.
In general, a positive indication that a stylus has passed through the `in-presence` boundary and that the workstation is responding to his stylus positioning movements would be extremely useful to the user. The conventional way of indicating the `in-presence` condition is by illuminating a light emitting diode on a control box or to give visual feedback on a display screen. This is, however, not very useful especially under circumstances where the user is concentrating his attention on the position of the stylus tip. An alternative to visual feedback is to use auditory feedback but this has the disadvantage that it can prove to be very distracting for applications requiring multiple pointings per minute or when there are multiple workstations in close proximity to each other. It is also usual to reserve auditory feedback for the workstation response to a tip switch closure or a successful correlate operation.
Tactile feedback is another alternative which has been used very successfully for keyboards and it is this technique which has been applied in the present invention to provide a response to the user indicating the instant in time when a stylus tip crosses the invisible `in-presence` boundary of the tablet.